precursors are fundamental for all aspects of climate change analysis. Emissions data are needed to drive climate models of the atmosphere, cryosphere, oceans, and biosphere. Scenarios of human-caused emissions are central to policy analysis of climate change because they provide future baseline trends for emissions that policy seeks to alter. The major driving forces of future emissions, such as demographic patterns, economic development, and environmental conditions, also underpin the assessment of vulnerability and development of adaptation strategies.

The available approaches for estimating future emissions to the atmosphere from human activities are described in this section. The methods used to relate emissions to atmospheric concentrations of greenhouse gases and aerosols are discussed in Chapter 6. A category of emissions that is not addressed in detail is volcanic eruptions. Estimating future volcanic emissions is not possible due to limitations in the understanding of the causes of volcanic eruptions, whose frequency depends on the rates of plate motions and of formation of large igneous provinces (see Chapter 3). One can only say, based on past history, that large climate-impacting eruptions will most likely occur, but it is not possible to predict when.

Anthropogenic Emissions

Projecting future emissions differs from other types of prediction that scientists make. Many natural systems, such as planetary motions, are governed by well-understood physical natural laws. The ability to predict future behavior and even events can be impressively high. Other physical systems may have well-understood physical laws, but these laws have shown that long-term prediction is impossible (e.g., chaotic systems). A third category includes systems to which the concept of governing laws expressed by mathematical equations is not applicable (Gaffin, 2002). For these systems, the driving forces change over time, sometimes radically, making prediction of specific outcomes a speculative effort. Many social, political, and economic science systems would fall into this category. Future anthropogenic emissions are a function of such socioeconomic systems and, as a result, are inherently unpredictable. Instead, emissions analysts offer “scenarios” that illustrate possible pathways for future emissions. These scenarios are not ruled out by current understanding of the driving forces behind emissions (Nakićenović et al., 2000). Figures 5-1 and 5-2 show scenarios for carbon dioxide (CO2) and sulfur dioxide (SO2) emissions.

In developing scenarios, it has been standard practice to consider futures with a range of policy interventions. On one end of the spectrum are noninterventionist emissions scenarios, which have been variously named business as usual, normative, reference, and no climate policy, among other terms. These scenarios seek to answer the policy question, How would

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